You probably set up a battery reaction with the steel underneath the plates.
You can do several things:
Don't use conductive screws to attach it (or use bushings), and use a sheet of mylar between the metals to protect the paint.
Etch the aluminum oxide off with acid, polish, and then clearcoat the aluminum to keep it from oxidizing again
Or, have them anodized.
Aluminum oxide is very very hard and almost impossible to grind off with abrasives, you need to use chemical means.
